Audio time synchronization using prioritized schedule

ABSTRACT

A method is provided for synchronizing playback of audio an/or video by a plurality of separate devices in a computer network, e.g. in a wi-fi network. Each separate device is programmed to select a synchronization mechanism in accordance with a predetermined prioritized list of at least two different synchronization mechanisms, and to use the selected synchronization mechanism for synchronizing audio and/or video playback. E.g. use of a clock based on the audio codec clock can be set to a higher priority than use of the system clock, which provides a poorer precision. A session leader serves to provide the synchronization to other separate devices in a session, however a group of two or more separate devices within the session may agree on selecting a synchronization mechanism providing a higher precision than the one provided by the session leader. E.g. to allow high precision timing between separate left and right loudspeakers in a stereo setup. A group leader can be elected to provide synchronization to a group of devices using a higher synchronization precision than the synchronization mechanism provided by the overall session leader. E.g. a dedicated synchronization channel separate from the audio/video streaming channel may be selected.

FIELD OF THE INVENTION

The present invention relates to the field of synchronized playback of audio or video between devices in a peer-to-peer computer network system, e.g. in a wi-fi network.

BACKGROUND OF THE INVENTION

In synchronized playback of audio or video between a plurality of devices in a peer-to-peer computer network system, the devices can participate in coordinated sessions, where the participating devices can play synchronously. The participating devices can be located within or across several rooms.

One of the key parameters of providing precise audio playback synchronization is the ability to maintain a common reference time between multiple loudspeakers in a peer-to-peer audio playback network system. E.g. precise timing between left and right stereo loudspeaker in a stereo setup is crucial in order to obtain a perceived stereo image without any perceivable displacement of the spatial location of the center of the stereo image. Even timing differences below hundreds of microseconds between left and right loudspeaker in a stereo setup may result in a disturbed stereo image. Further, synchronization is required of audio and video between e.g. a loudspeaker device playing the audio part and a display device, e.g. a TV set, showing the video part of audio/video material. The synchronization requirements between the TV and the loudspeakers are orders-of-magnitude lower than the requirements between stereo loudspeakers.

In order to playback synchronously, the participating devices need to agree on the playback time and pace. One of the key parameters of providing precise audio playback synchronization is the ability to share the same time between multiple devices in a network, especially a wireless network.

Solutions exist that have various means to achieve synchronized audio playback in wireless computer networks. Some of these systems use the system clock of the devices, others provide a hardware solution requiring each of the nodes in the network to run with dedicated hardware. Even when working without adjusting the system clocks as provided by the operating system, these solutions often use clocks that are derived from the system clock, adjusted with a frequency and offset component to match some remote system. However, such synchronization is in general not accurate.

SUMMARY OF THE INVENTION

Thus, according to the above description, it is an object of the present invention to provide an accurate way of providing synchronized playback of audio and/or video in a computer network of a plurality of devices.

In a first aspect, the invention provides a method for synchronizing playback of audio and/or video from a source in computer network of a plurality of separate devices in a computer network session, see appended claim 1.

In this method, the best-effort session synchronization mechanism is selected from a hierarchy of heterogeneous synchronization mechanisms. In other words, a prioritized schedule is used to select the synchronization mechanism to be used.

Such method provides a general way of allowing different types of devices to be combined to playback time synchronous audio and/or video in a computer network session, and to still obtain the most precise time synchronization possible.

A session is directed by a session master (or session leader) which has a lowest common denominator synchronization mechanism. Subsets of participating devices may group together to use a better synchronization mechanism within the group, with a group master that synchronizes the group with the session master (session leader).

By using different synchronization mechanisms for different separate devices in a session, it is possible to achieve a better synchronization between devices with higher synchronization requirements, such as between the left and right loudspeakers for stereo audio playback. In comparison, the synchronization between two devices located in different rooms is relaxed, and can thus be implemented by another synchronization mechanism.

In the dependent claims 2-20, a number of embodiments and/or preferred features will be defined.

By ‘audio clock’ is understood a synchronization mechanism provided by the audio codec used to stream the audio of the audio and/or video content.

In a second aspect, the invention provides a protocol for controlling a plurality of separate devices in a computer network, see appended claim 21.

In a third aspect, the invention provides a computer executable program code, or a programmable- or fixed hardware, and/or combination hereof, arranged to perform the method according to the first aspect, or causes a device with a processor to function according to the protocol of the second aspect. The computer executable program code may be stored on a data carrier. The program code may be implemented to function on any type of processor platform.

In a fourth aspect, the invention provides a device that operates according to the method or protocol of the first or second aspect. Especially, the device may be an audio device, such as a stand alone active loudspeaker. The device may also be any other type of device with audio and/or video capabilities, e.g. a TV set.

In a fifth aspect, the invention provides a system of a plurality of devices according to the fourth aspect, e.g. mixed types of devices arranged to take part in a computer network session, e.g. a wi-fi network session, of synchronized playback of audio and/or video content.

In a sixth aspect, the invention provides an electronic chip programmed to allow a device to operate according to protocol according to the third aspect, if provided with access to the electronic chip, e.g. if the device has the chip installed therein.

It is appreciated that the same advantages and embodiments described for the first aspect apply as well for the second, third, fourth, fifth, and sixth aspects. Further, it is appreciated that the described embodiments can be intermixed in any way between all the mentioned aspects.

BRIEF DESCRIPTION OF THE FIGURES

The invention will now be described in more detail with regard to the accompanying figures of which

FIG. 1 shows a synchronization session with a TV (10) and a stereo set of loudspeakers, left and right (20 and 21), where the TV (10) is the session master (leader) and therefore provides the overall synchronization (30 and 31),

FIG. 2 illustrates a synchronization mechanism selection algorithm embodiment.

FIG. 3 illustrates an example of a synchronization hierarchy. The session master (leader) (50) is the authoritative time synchronization source for the session, and it provides timing information to all other devices (52), (54), and (56) via the session's default synchronization. Two groups have been formed with an out-of-band synchronization, each group having a group master (leader) (52) and two group members (54).

FIG. 4 illustrates steps of a clock synchronization selection algorithm embodiment.

The figures illustrate specific ways of implementing the present invention and are not to be construed as being limiting to other possible embodiments falling within the scope of the attached claim set.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 shows a synchronization session with a TV (10) and two speakers (20 and 21). The TV (10) is the session master and therefore provides the overall synchronization (30 and 31). The Right speaker (21) and the Left speaker (20) form a group (40) wherein the Right speaker (21) acts as the group master. The group master (21) synchronizes against the session master (10). The Left speaker (20) ignores the synchronization from the session master (10) and instead uses the synchronization (32) from the group master (21).

FIG. 2 illustrates a synchronization mechanism selection algorithm.

FIG. 3 illustrates a synchronization hierarchy: The session master (50) is the authoritative time source for the session. It provides timing information to all other devices (52), (54), and (56) via the session's default synchronization. Two groups have been formed with an out-of-band synchronization. Each group has a group master (52) and two group members (54). These use an out-of-band synchronization mechanism. As indicated by the dashed line, (54) still utilizes the default synchronization with the session master (50).

The group masters (52) also maintain a direct synchronization with the Session Master (50). Devices (56) do not use any out-of-band synchronization, and fall back to the default synchronization with the session master (50).

FIG. 4 illustrates a simple example of a selection algorithm implemented in each of the separate devices taking part in a computer network session of synchronous playback of audio and/or video.

In a specific example, the prioritized list of synchronization mechanisms may include two or more of the following, in prioritized order:

1) a custom radio frequency (RF) based synchronization mechanism,

2) a custom infrared (IR) based synchronization mechanism,

3) a Global Positioning System (GPS) based synchronization mechanism,

4) a standard radio clock (e.g. DCF77 or WWVB) based synchronization mechanism, and

5) a Network Time Protocol (NTP) based synchronization mechanism.

To sum up: the invention provides a method for providing a synchronization in a computer network for synchronized playback of audio and/or video by a plurality of separate devices, e.g. in a wi-fi network. Each separate device is programmed to select a synchronization mechanism in accordance with a predetermined prioritized list of at least two different synchronization mechanisms, and to use the selected synchronization mechanism for synchronizing audio and/or video playback. E.g. use of a clock based on the audio codec clock can be set to a higher priority than use of the system clock, which provides a poorer precision. A session leader serves to provide the synchronization to other separate devices in a session, however a group of two or more separate devices within the session may agree on selecting a synchronization mechanism providing a higher precision than the one provided by the session leader. E.g. to allow high precision timing between separate left and right loudspeakers in a stereo setup. A group leader can be elected to provide synchronization to a group of devices using a higher synchronization precision than the synchronization mechanism provided by the overall session leader. E.g. a dedicated synchronization channel separate from the audio/video streaming channel may be selected.

Although the present invention has been described in connection with the specified embodiments, it should not be construed as being in any way limited to the presented examples. The scope of the present invention is to be interpreted in the light of the accompanying claim set. In the context of the claims, the terms “including” or “includes” do not exclude other possible elements or steps. Also, the mentioning of references such as “a” or “an” etc. should not be construed as excluding a plurality. The use of reference signs in the claims with respect to elements indicated in the figures shall also not be construed as limiting the scope of the invention. Furthermore, individual features mentioned in different claims, may possibly be advantageously combined, and the mentioning of these features in different claims does not exclude that a combination of features is not possible and advantageous. 

1. A method for synchronizing playback of audio and/or video from a source in a network of a plurality of separate devices in a computer network session, wherein each of the separate devices in the computer network session are programmed to: select a synchronization mechanism in accordance with a predetermined prioritized list of at least two different synchronization mechanisms, so as to obtain the most precise time synchronization possible, wherein the prioritized list comprises a virtual clock generated in response to a timing of the audio codec used for streaming the audio and/or video to select from, and use the selected synchronization mechanism for synchronizing audio and/or video playback.
 2. Method according to claim 1, wherein one of the separate devices in the computer network session is selected as session leader, wherein the session leader is programmed to provide a default synchronization mechanism which is selected to be the synchronization mechanism with the lowest priority on said predetermined prioritized list of at least two different synchronization mechanisms, and wherein the separate devices in the computer network session are programmed to select a synchronization mechanism in accordance with a predetermined selection criterion.
 3. (canceled)
 4. Method according to claim 1, wherein the separate devices are programmed to search within the computer network session for other separate devices, and to form a group of at least two separate devices within the computer network session to agree on a synchronization mechanism with a higher priority than used by other separate devices in the computer network session, so as to obtain a higher synchronization precision between the separate devices being part of the group.
 5. Method according to claim 4, wherein one of the separate devices in said group is elected as group leader to provide synchronization according to the selected synchronization mechanism with higher priority synchronization mechanism, and wherein the group leader serves to synchronize group with the synchronization provided by the session leader, and wherein said group comprises an audio device arranged to playback left stereo channel of an audio stream, and another audio device arranged to playback right stereo channel of the audio stream. 6-7. (canceled)
 8. Method according to claim 1, wherein said predetermined prioritized list comprises at least one synchronization mechanism which operates within the communication channel on which the audio and/or video content is streamed.
 9. Method according to claim 1, wherein said predetermined prioritized list comprises at least one synchronization mechanism which operates on a communication channel separate from the communication channel on which the audio and/or video content is streamed.
 10. Method according to claim 9, wherein the audio and/or video content is streamed via a wi-fi channel, and wherein synchronization is provided via a separate wireless channel, such as a separate wireless channel communication implemented by dedicated hardware in the separate devices.
 11. Method according to claim 1, wherein said predetermined prioritized list comprises at least the following types of synchronization mechanisms to so select from: 1) use of the audio clock as synchronization mechanism, and 2) use of the system clock in each of the separate devices as synchronization mechanisms.
 12. (canceled)
 13. Method according to claim 1, wherein said predetermined prioritized list comprises at least two of: 1) a custom radio frequency (RF) based synchronization mechanism, 2) a custom infrared (IR) based synchronization mechanism, 3) a GPS based synchronization mechanism, 4) a standard DCF77/WWVB RF based synchronization mechanism, and 5) an NTP-to-public server based synchronization mechanism.
 14. Method according to claim 1, wherein said predetermined prioritized list comprises at least three types of synchronization mechanisms to select from.
 15. (canceled)
 16. Method according to claim 1, wherein said virtual clock is generated in response to a clock system of the audio codec, such as by direct access to the tick counter of the audio codec clock system, or wherein the virtual clock is generated in response to periodic measurements of the audio codec clock.
 17. Method according to claim 1, wherein synchronization in said session is provided by one of the separate devices serving as session leader, and wherein each of the separate devices are programmed to elect a new session leader among the remaining separate devices in the session in an autonomous manner, in case it is detected that the current session leader is unavailable.
 18. (canceled)
 19. A protocol for controlling a plurality of separate devices in a computer network, wherein the protocol implemented in each of the separate devices in the computer network session comprises: selecting a synchronization mechanism in accordance with a predetermined prioritized list of at least two different synchronization mechanisms, so as to obtain the most precise time synchronization possible, wherein the prioritized list comprises a virtual clock generated in response to a timing of the audio codec used for streaming the audio and/or video to select from, and using the selected synchronization mechanism for synchronizing audio and/or video playback.
 20. A computer executable program code arranged to perform the method according to claim
 1. 21. (canceled)
 22. A device comprising a processor and a network interface arranged to receive audio and/or video via a computer network from a source in a synchronized manner in response to synchronization information, such as synchronization information from a session leader, wherein the processor is programmed to operate according to the method according to claim 23-27. (canceled)
 28. Method according to claim 1, wherein said predetermined prioritized list comprises a custom radio frequency based on synchronization mechanism with a higher priority than the virtual clock generated in response to a timing of the audio codec used for streaming the audio and/or video. 